It is known that atmospheric air contains the compounds or impurities that have to be removed before any cryogenic separation of the air, that is to say prior to introducing the air into the heat exchangers of the cold box of an air separation unit.
In particular, mention may be made of the compounds of the carbon dioxide (CO.sub.2) and/or water vapour (H.sub.2 O) type, but also of other impurities.
This is because, in the absence of such a pretreatment of the air, these impurities, particularly CO.sub.2 and/or water vapour, would inevitably condense and solidify as ice while the air is being cooled to a cryogenic temperature, something which would cause problems of blocking in the cryogenic separation equipment or unit, especially the heat exchangers, distillation columns, etc., and thereby cause the equipment or unit to be damaged.
To avoid these problems, it is common practice to pretreat the air that has to be cryogenically separated before this cryogenic separation.
This pretreatment of the air is usually called "front end" scrubbing or purification since it is carried out upstream of the cryogenic separation unit.
Currently, the air is pretreated by a TSA (Temperature Swing Adsorption) process or by a PSA (Pressure Swing Adsorption) process depending on the case.
Conventionally, a TSA process cycle comprises the following steps:
a) purification of the air by adsorption of the impurities at superatmospheric pressure; PA1 b) depressurization of the adsorber down to atmospheric pressure or below atmospheric pressure; PA1 c) complete regeneration of the adsorbent at atmospheric pressure using a hot gas, especially the residual gases or waste gases, typically impure nitrogen coming from an air separation unit and heated by means of one or more heat exchangers; PA1 d) cooling of the adsorbent, especially by continuing to introduce into it the residual gas coming from the air separation unit, but not heated; PA1 e) repressurization of the adsorber using the purified air, coming, for example, from another adsorber which is in production phase. PA1 (a) compressing the air to be separated to a pressure of at least 11.4.times.10.sup.5 Pa; PA1 (b) introducing the air at a temperature greater than or equal to +15.degree. C. and at a pressure of at least 11.4.times.10.sup.5 Pa in at least one adsorption vessel containing particles of at least one adsorbent; PA1 (c) adsorbing at least some of the impurities contained in the air on the particles of adsorbent at a pressure of at least 11.4.times.10.sup.5 Pa; PA1 (d) cooling the air purified in step (c) down to a cryogenic temperature of less than -120.degree. C., preferably down to a temperature of less than -170.degree. C.; PA1 (e) cryogenically distilling the air cooled in step (d). PA1 (a) compressing the air to a pressure of at least 11.4.times.10.sup.5 Pa; PA1 (b) introducing the compressed air into at least one adsorption vessel containing particles of at least one adsorbent; PA1 (c) adsorbing at least some of the impurities contained in the air on the particles of adsorbent at a pressure of at least 11.4.times.10.sup.5 Pa and at a temperature of at least +15.degree. C.; PA1 (d) cooling the air purified in step (c) down to a cryogenic temperature of less than -120.degree. C., preferably down to a temperature of less than -170.degree. C.; PA1 (e) cryogenically distilling the air cooled in step (d). PA1 after cryogenic distillation, at least one compound chosen from nitrogen, oxygen, argon or their mixtures is recovered. The compound or compounds thus produced may be of variable purity and at least one of these compounds is preferably recovered in liquid form in order to utilize the energy provided by the increase in the pressure of the air; PA1 in step (c), the adsorption is carried out at a pressure of at least 12.times.10.sup.5 Pa, preferably at least 15.times.10.sup.5 Pa, at least 20.1.times.10.sup.5 Pa, preferably at least 20.5.times.10.sup.5 Pa, and preferably at a pressure ranging from 21.times.10.sup.5 Pa to 40.times.10.sup.5 Pa, advantageously from 22.times.10.sup.5 Pa to 38.times.10.sup.5 Pa; PA1 the compressed air is not precooled after step (a) and before step (b); PA1 in step (a), the compression is carried out at a pressure of at least 12.times.10.sup.5 Pa, preferably at least 15.times.10.sup.5 Pa, at least 20.1.times.10.sup.5 Pa, preferably at least 20.5.times.10.sup.5 Pa, and preferably at a pressure ranging from 21.times.10.sup.5 Pa to 40.times.10.sup.5 Pa, advantageously ranging from 22.times.10.sup.5 Pa to 38.times.10.sup.5 Pa; PA1 in step (b), the air introduced into the adsorption vessel is at an adsorption temperature of at least +21.degree. C., preferably ranging from +25.degree. C. to +65.degree. C., advantageously from +30.degree. C. to +50.degree. C. When the air leaving the compression step is at too high a temperature, for example +80.degree. C., it is cooled in order to bring it into a temperature range of the process according to the invention but, in all cases, to a temperature of at least +15.degree. C. In other words, the process of the invention may optionally include a complementary step of adjusting the temperature of the air, after step (a) and before step (b), but without accordingly constituting a conventional precooling step using a refrigeration unit. Advantageously, this complementary step of adjusting the temperature of the air simply consists in decreasing the temperature of the air by heat exchange by means of one or more heat exchangers, preferably exchangers through which water circulates; PA1 in step (c), the adsorbed impurities are chosen from the group formed by carbon dioxide, water vapour, nitrogen oxides, sulphur oxides and hydrocarbons; PA1 the adsorbent is chosen from zeolites, aluminas, silica gels and mixtures thereof; PA1 the cryogenic distillation of step (e) is carried out at a pressure ranging from 10.sup.5 Pa to 30.times.10.sup.5 Pa, preferably about 1.3.times.10.sup.5 Pa to 20.times.10.sup.5 Pa; PA1 it furthermore includes a step of regenerating the adsorbent at a regeneration temperature of between 40.degree. C. and 350.degree. C., preferably between 100.degree. C. and 280.degree. C. In a manner known per se, the adsorbent is cylically regenerated by heating it to the regeneration temperature by flushing the adsorbent particles with a gas at the desired regeneration temperature, preferably a gas chosen from nitrogen or a mixture of nitrogen and oxygen; PA1 the rate of regeneration is equal to 5 to 70% of the rate of adsorption; PA1 the rate of adsorption is between 250 Sm.sup.3 /h and 10.sup.6 Sm.sup.3 /h; PA1 the regeneration pressure is between 1.times.10.sup.5 Pa and 30.times.10.sup.5 Pa; PA1 in step (c), the adsorption of the impurities contained in the air is carried out in a TSA-type cycle; PA1 the particles of adsorbent are chosen from zeolite particles of the faujasite, X, LSX, A or mordenite type, or mixtures thereof, and preferably the zeolite particles are of the X or LSX type having an Si/Al ratio.ltoreq.1.15, preferably approximately 1; PA1 the particles of adsorbent are chosen from particles of activated alumina and of impregnated alumina; PA1 at least one adsorber contains at least one bed or at least one layer of zeolite particles and at least one bed or at least one layer of alumina particles and preferably at least one bed or at least one layer of alumina particles is located upstream of at least one bed or at least one layer of zeolite particles with respect to the direction of flow of the air. PA1 at least one air compression means for compressing the air to a pressure of at least 11.4.times.10.sup.5 Pa, preferably an air compressor; PA1 at least one heat exchanger to adjust, if necessary, the temperature of the air to a value of at least +15.degree. C., preferably of between +30.degree. C. and +50.degree. C.; PA1 at least one adsorber, preferably at least two adsorbers arranged in parallel, containing at least one bed or at least one layer of particles of at least one adsorbent; PA1 at least one cold box for cooling the purified air leaving at least one adsorber down to a cryogenic temperature of less than approximately -120.degree. C.; and PA1 at least one cryogenic distillation column for separating, by cryogenic distillation, the air cooled in the cold box, PA1 (a) compressing the air to a pressure of at least 11.4.times.10.sup.5 Pa, preferably to at least 15.times.10.sup.5 Pa; PA1 (b) introducing the compressed air into at least one adsorption vessel containing particles of at least one adsorbent; PA1 (c) adsorbing at least some of the impurities contained in the air on the said particles of adsorbent at a pressure of at least 11.4.times.10.sup.5 Pa, and at a temperature of at least +15.degree. C., preferably at at least +21.degree. C.; PA1 (d) recovering the air purified in step (c).
Moreover, as regards a PSA process cycle, this comprises substantially the same steps a), b) and e), but is distinguished from a TSA process by the residual gas or gases not being heated during the regeneration step (step c)), and therefore by the absence of step d) and, in general, a shorter cycle time than in a TSA process.
Preferably, the air pretreatment devices comprise at least two adsorbers, operating in parallel, that is to say alternately, one of the adsorbers being in production phase while the other is in regeneration phase.
Such TSA or PSA air purification processes are described, for instance, in documents U.S. Pat. No. 3,738,084, 5,531,808, 5,587,003 and 4,233,038.
However, it is known that, when particles of adsorbent are used to prepurify the air before it is separated by cryogenic distillation it is common practice to adjust (by water cooling) the temperature of the compressed air from a temperature usually of at least 80.degree. C., or even higher, down to room temperature and then subsequently to precool the air before it is introduced into the adsorber or adsorbers, this usually being carried out by a refrigeration unit which cools the air from room temperature down to a temperature below room temperature.
This is, moreover, clearly explained in the document "Industrial Gases & Cryogenics Today, IOMA Broadcaster, Air Purification for cryogenic air separation units, January-February 1984, p. 15 et seq." or by the document EP-A-438,282.
In fact it is particularly recommended to precool the air before subjecting it to an adsorption separation step since, as is known by those skilled in the art, the lower the adsorption temperature the more effective the adsorption of the impurities.
In other words, the effectiveness of the air prepurification is markedly improved at low temperatures, that is to say temperatures close to 5.degree. C., or improved even more at markedly lower temperatures.
Next, after its passage through the purification zone, that is to say in the adsorber or adsorbers, the air stripped of all or some of its deleterious impurities, especially of the CO.sub.2 and H.sub.2 O type, is then conventionally cooled to a cryogenic temperature, that is to say generally to a temperature of less than approximately -120.degree. C., before being sent into the cryogenic distillation unit and introduced into one or more distillation columns for the purpose of being separated therein in order to recover the nitrogen, oxygen and/or argon.
However, the fact of having to employ an air precooling step before the air is introduced into the adsorber or adsorbers has several drawbacks which have a negative impact on the industrial advantage of the overall process.
This step of precooling the air has the effect of appreciably increasing the overall investment, of complicating the process, of possibly causing reliability problems and therefore of resulting in an additional cost for the plant, given that it is then necessary to provide cooling means, such as heat exchangers or the like, that is to say a refrigeration unit.
Mention may be made of the documents U.S. Pat. No. 4,249,915, EP-A-733393, EP-A-718576, U.S. Pat. No. 5,463,869 and JP-A-54103778 describing various processes for the treatment of the air before it is cryogenically separated for the purpose of producing nitrogen and oxygen.